Wednesday, 4 February 2015

A widely-used brain stimulation technique may be less effective than previously believed.

Transcranial Direct Current Stimulation (tDCS) is an increasingly popular neuroscience tool. tDCS involves attaching electrodes to the scalp, through which a weak electrical current flows. The idea is that this current modulates the activity of the brain tissue underneath the electrode - safely and painlessly.

However, according to Australian neuroscientists Jared Horvath, Jason Forte and Olivia Carter, a single session of tDCS may have no detectable effect on cognitive function in most people. In a new paper published in the journal Brain Stimulation, Horvath and colleagues reviewed the published evidence on tDCS. They performed a meta-analysis of the data on how tDCS influences cognitive functions such as memory, language, and mental arithmetic.

For example, in experiments investigating language function, neuroscientists generally place the active tDCS electrode over the left frontal lobe of the volunteers. This ensures that the electrode is near to Broca’s area, a part of the brain known to be involved in language production. Then, the current is switched on and the volunteer is asked to do a linguistic task such as verbal fluency, in which the goal is to think of as many words beginning with a certain letter (say “p”) as possible within one minute. The performance of the volunteers given tDCS is compared to the performance of people given “sham” tDCS, in which the electrodes are attached but no current is applied.

Horvath et al. found that overall, there was no statistically significant difference between active and sham tDCS on any of the cognitive tasks that they examined. They say that:

Of the 59 analyses undertaken, tDCS was not found to generate a significant effect on any. Taken together, the evidence does not support the assertion that a single-session of tDCS has a reliable effect on cognitive tasks in healthy adult populations.

That seems pretty clear-cut. However, Horvath et al. acknowledge that their analysis did not include any of the studies that have been conducted on individuals with brain diseases or on the elderly, and they note that tDCS might be more effective in such cases.

What’s more, Horvath et al.’s meta-analysis didn’t utilize all of the studies on healthy people. The authors decided to only include results that had at least one published independent replication attempt. In other words, they only included studies that had measured the effects of tDCS on a given cognitive task, if more than one different research group had published papers using that technique. Even if one team of scientists had published several studies all showing that tDCS does influence some aspect of cognition, those results weren’t included unless at least one other team of researchers had published tDCS results using that same task. One hundred and seventy-six articles were excluded as a result.

Horvath et al. explain their decision not to consider those studies by saying that:

We chose to exclude measures that have only been replicated by a single research group to ensure all data included in and conclusions generated by this review accurately reflect the effects of tDCS itself, rather than any unique device, protocol, or condition utilized in a single lab.

However, this is a slightly unusual restriction to use on a meta-analysis. It might be interesting to see whether including these additional studies would have changed the results.

This is the second time Horvath, Forte and Carter have published a sceptical meta-analysis of tDCS. In November last year they reviewed studies on the neurophysiological effects of tDCS and concluded that tDCS has virtually no measurable effects on brain function. So Horvath et al. seem to have comprehensively shown that tDCS essentially has no impact in healthy people, either on a biological or on a cognitive level.

This is a really useful review, as it helps us to think about the way we talk about the effects of tDCS.

However I believe that the way the analysis was conducted may have obscured some of the very real effects of tDCS. The authors have made a judgement about which studies can be pooled together and which studies cannot be pooled. One always has to make these kinds of decisions and I am not sure I would have made the same decisions given the same choices.

tDCS is still a developing technology. I think that with more principled methods of targeting the current flow to the desired brain area, we will see tDCS become one of the standard tools of cognitive neuroscience, just as EEG and fMRI have become.